Normally open sensors are used in a variety of situations to provide status information of components. For example, in a helicopter, a normally open sensor closes an electrical circuit when the sensor is activated. The electrical circuit is connected to an indicator, such as a warning light on the pilot's instrument panel in the cockpit. The sensor is typically located remote from the instrument panel. Consequently, the sensor is connected to the indicator panel by wires, connectors, etc. Any element along this path could experience an electrical fault. Because the sensor is normally an open circuit, detection of a fault has been difficult.
An example of a normally open sensor is a chip detector. A chip detector is used to monitor the health or air worthiness of a helicopter's transmission or gear box, which is a vital piece of equipment. The presence of a significant number of metal chips in the transmission fluid usually indicates mechanical problems with the transmission. The chip detector is partially immersed in the transmission fluid so as to be exposed to the metal chips circulating inside of the fluid. The chip detector is provided with a magnet so as to attract and retain the metal chips. The presence or absence of metal chips captured by the chip detector is indicated both visually and electrically. The electrical indication is provided by a warning light on the instrument panel. If metal chips accumulate during flight, the warning light is illuminated and the pilot can safely land the helicopter, before the rotors lock up. However, if the circuit between the chip detector and the instrument panel experiences a fault, detection is made difficult by the open circuited nature of the detector. The visual indication occurs when the chip detector is pulled from its hole in the transmission. accumulation of metal chips can be seen.
U.S. Pat. Nos. 5,250,909 and 5,583,441 show an apparatus for detecting faults in electrical circuits containing normally open devices such as chip detector. The chip detector is provided with a resistive device across the open circuit. The resistive device provides a path for current through the electrical circuit. The continuity of the circuit can be monitored, without interfering with the sensing function of the chip detector.
The resistive device is a discrete component, such as a resistor, that is added to the chip detector during the manufacture of the chip detector. For example, the resistor can be located adjacent to the contacts. This involves electrically connecting the conductive leads of the resistor to the contacts. Alternatively, the resistor can be located inside of the chip detector housing. Such methods of installing a resistor in a chip detector are sometimes laborious. In addition, if the chip detector is utilized on an aircraft, then modifying the chip detector with the resistor requires qualification by regulatory authorities.
It is desirable to provide this diagnostic capability in an inexpensive and simple manner.
In addition, wiring harnesses in aircraft can be especially difficult to access for inspection. If there is a fault along the wiring harness, it is helpful to locate that fault. For example, locating the fault could place it between two bulkheads. If the circuit is not critical, or if the fault is located away from a critical item (for example, a fuel tank), then the circuit can be switched off and the aircraft maintained in service.
In the prior art, there are devices that, once connected to the wires, locate faults. However, these devices necessitate taking the circuit out of service. Such testing must be done on the ground during aircraft maintenance. It is desirable to provide fault location capabilities in real time, as the aircraft is in service.